1. Field of the Invention
The present invention relates to a disposable glucose strip sensor configured to rapidly and conveniently measure the concentration of glucose in blood and a glucose measurement method using the glucose strip sensor.
2. Description of the Related Art
The measurement of the concentration of glucose in blood is of great importance not only to diabetic patients who must control their sugar intake, but also for the early detection and diagnosis of diabetes. To this end, methods for simply and conveniently measuring the concentration of glucose in blood have been proposed.
Known glucose measurement methods are based on oxidation of glucose by glucose-oxidase and peroxidase. They also use orthotolidine or a benzidine-based mixture as an indicator reagent, that is, a chromogen. In accordance with these methods, a color transition of the indicator reagent resulting from the oxidation of glucose is observed to measure the concentration of glucose in blood.
For example, such techniques are disclosed in U.S. Pat. No. 3,061,523 and Japanese Patent Publication No. Sho. 50-39558. In these references, a glucose-measuring test piece is disclosed. In order to prepare this test piece, a solution is prepared which has a composition including: glucose oxidase and peroxidase as enzymes; a citric acid buffer to maintain a pH of 6.0; gelatin, alginic acid, polyvinylpyrrolidone, and polyvinyl alcohol as stabilizers; and orthotolidine, benzidine, 3-aminopropylcarbarsone, and 2,7-diaminofluorene as a chromogen. The solution is impregnated into a cellulose paper which has a desired thickness and size to be used as a carrier, and then dried. Thus, the test piece is obtained. Also, Korean Patent Laid-open Publication No. 85-1297 discloses a method for manufacturing a glucose-measuring test piece, to which the basic principle of an enzymatic measurement method using glucose oxidase and peroxidase is applied. Where the concentration of glucose in blood is measured using the above mentioned glucose-measuring test pieces, it is difficult to accurately measure a glucose concentration because the measurement is based on a color transition exhibited on the test piece.
In order to solve the above mentioned problem, various techniques have been proposed which measure glucose concentration using an electrochemical method. Such an electrochemical method makes it possible to measure the concentration of glucose in blood with an increased accuracy while reducing measurement time and achieving convenience in measurement. By virtue of such advantages, the use of the electrochemical glucose measurement method has been greatly increased.
Now, the operating principle of a glucose-measuring sensor based on an electrochemical method will be described. When a blood sample is applied to a reaction layer of the glucose-measuring sensor, glucose contained in the blood sample is oxidized by a glucose-oxidizing enzyme contained in the reaction layer. At this time, the glucose-oxidizing enzyme is reduced. The reduced glucose-oxidizing enzyme is then oxidized by an electron acceptor, whereby the electron acceptor is reduced. The reduced electron acceptor donates electrons at the surface of an electrode to which a desired voltage is applied. As a result, the electron acceptor is electrochemically reoxidized. The concentration of glucose in the blood sample is proportional to the amount of current generated during the process in which the electron acceptor is oxidized. Accordingly, the concentration of glucose can be measured by measuring the amount of current.
An example of the above mentioned glucose-measuring sensor is disclosed in Japanese Patent Laid-open Publication No. 61-294351. This sensor is illustrated in FIG. 1. As shown in FIG. 1, operating and counter electrodes, which are made of carbon or the like, are formed on a substrate 111 in a screen printing fashion. An insulator 115 is also formed on the substrate 111 while allowing the electrodes to be partially exposed. A porous reaction layer 117, which contains a reactive material such as a glucose-oxidizing enzyme and an electron acceptor, is arranged on the insulator 115. In order to firmly hold the porous reaction layer 117, a holding frame 116 and a cover 118 are arranged on the insulator 115. In FIG. 1, reference numerals 112, 113, and 114 denote the operating and counter electrodes, and reference numerals 112′, 113′, and 114′ denote the exposed portions of the operating and counter electrodes. These electrodes and electrode portions form an electrode system. When a blood sample is dropped onto the porous reaction layer 117, the glucose-measuring sensor having the above mentioned structure can measure the concentration of glucose in the blood sample.
In this glucose-measuring sensor, however, the amount of blood absorbed in the reaction layer 117 varies depending on the amount of the blood sample dropped onto the reaction layer 117. As a result, measurement errors may be caused by a variation in the amount of blood absorbed in the reaction layer 117.
In order to solve this problem, a biosensor has been proposed. An example of such a biosensor is disclosed in U.S. Pat. No. 5,120,420 and illustrated in FIG. 2. As shown in FIG. 2, this biosensor includes a non-conductive substrate 211 made of polyethylene terephthalate. Silver is screen-printed on the non-conductive substrate 211 to form leads 212 and 213. Conductive carbon paste containing a resin binder is printed on the leads 212 and 213, thereby forming an operating electrode 214 and a counter electrode 215. An insulator 216 is then printed to allow the electrodes 214 and 215 to be partially exposed. A 0.5% aqueous solution of carboxymethyl cellulose (CMC) is spread onto the electrodes 214 and 215, and dried to form a CMC layer. A solution of glucose oxidase (GOD) as the enzyme in a phosphate buffer solution is spread on the CMC layer, and dried to form a main reaction layer comprised of a CMC-GOD layer. Next, a resin plate 217 and a cover 219 are attached to the resulting structure while defining a space 218. In FIG. 2, the reference numeral 220 denotes a sample introducing port, and the reference numeral 221 denotes a discharge port.
In the biosensor having the above mentioned structure, when a sample solution comes into contact with the sample introducing port 220, it is introduced into the space 218 by virtue of capillary phenomenon, so that it fills the space 218. Simultaneously, air existing in the space 218 is vented from the space 218 through the discharge port 221 formed opposite to the sample introducing port 220 or at the cover 219.
Where the discharge port 221 is arranged at the upper surface of the biosensor, measurement errors may occur when the user unintentionally touches the discharge port 221. For this reason, there is inconvenience in handling the biosensor. Furthermore, the user can check whether or not a sufficient amount of sample solution is introduced in the biosensor, only with the naked eye. So, the measurement may be carried out even when an insufficient amount of sample solution is filled in the biosensor. In this case, however, the detected glucose level may erroneously be lower than the actual glucose concentration.